Pulverized coal-fired boilers are often used by electric utility companies in the United States. Furthermore, as a result of recent governmental regulation, levels of NOx emissions from pulverized coal firing boilers have been set which cannot be exceeded, and subsequently, numerous efforts have been undertaken to develop and implement changes to older pulverized coal firing boilers to bring NOx emissions within prescribed limits.
Pulverized coal-fired boilers typically include either wall-fired furnaces or tangentially-fired furnaces. Furthermore, while each type of furnace utilizes combustion for the production of heat, the differences between these two types of furnaces in the delivery of air and fuel into the combustion chamber have resulted in widely differing approaches to improvements therein for the reduction of NOx emissions. Thus, for example, U.S. Pat. No. 5,417,564 to Briggs; U.S. Pat. No. 4,991,520 to Tsumura et al.; and U.S. Pat. No. 4,403,941 to Okiura et al. each represent specific improvements to wall-fired furnaces intended to lower NOx emissions.
In contrast, improvements in boilers having tangentially-fired furnaces are separately represented by U.S. Pat. No. 4,672,900 to Santalla et al.; U.S. Pat. No. 4,501,204 to McCartney et al.; U.S. Pat. No. 5,311,829 to Clark et al.; U.S. Pat. No. 5,343,820 to Marion; U.S. Pat. No. 5,315,939 to Rini et al.; U.S. Pat. No. 4,426,939 to Winship; U.S. Pat. No. 5,626,085 to Donais et al.; and U.S. Pat. No. 5,020,454 to Hellewell et al.
The present invention belongs to the latter group of references, as the present invention specifically relates to decreasing NOx emissions in tangentially-fired furnaces. In this regard, a tangentially-fired furnace 20 representative of the state-of-the-art is shown in FIG. 1 and comprises a vertical combustion chamber 22 that is generally rectangular in cross-section and that includes in each of the four corners thereof a plenum chamber known as a windbox 24. For clarity of illustration, only two such windboxes 24 are shown in FIG. 1. Each windbox 24 is comprised of coal compartments through which passes coal and primary air for initial combustion, and auxiliary air compartments through which passes additional air for maintaining combustion. Furthermore, each windbox 24 and, specifically, each coal nozzle of each windbox 24, is located at least ten feet from a coutant bottom 26 of the furnace. The furnace also includes separate overfire air (SOFA) compartments 28 disposed above and separate from the windboxes 24 for introduction of additional air into the top of the combustion chamber for completing the final stages of combustion. The additional air can be introduced in opposition to the air flow within the furnace as shown, or introduced with the existing air flow within the furnace as is more common. In either case, use of SOFA compartments in tangentially-fired furnaces has been shown to reduce NOx emissions in tangentially-fired furnaces.
Conventional wisdom holds that by dividing the total amount of air (oxygen) between both the windboxes and the SOFA compartments, which otherwise would pass entirely through the windboxes, oxygen concentrations at each stage of combustion in the furnace is decreased. The decreased levels of oxygen, in turn, reduces the oxygen available for formation of NOx at each stage of combustion which, consequently, reduces the overall NOx produced. Conventional wisdom also holds that the reduction of oxygen at each stage of combustion further reduces the temperature at each stage of combustion which, in turn, reduces NOx produced at each stage of combustion as well as the overall NOx produced.
A conventional windbox 30 itself is shown in detail in FIG. 2 and comprises a vertical array of adjacent coal compartments 32 separated from each other by an auxiliary air compartment 34. Each coal compartment 32 includes a coal delivery pipe 36, a coal nozzle 38, and a coal tip 40 also known as a coal bucket. Pulverized coal carried in an air stream of primary air is delivered into the combustion chamber via the coal delivery pipe 36 and coal nozzle 38 and is directed into the combustion chamber by the coal tip 40. Air openings 42 are also provided in the coal compartments 32 for introduction of air, known as "fuel air," in direct proximity to the primary air to assist in the initial combustion of the coal. The auxiliary air compartments 34 disposed between the coal compartments 32 in the windbox 30 include air nozzles 44 for introduction of the auxiliary air into the combustion chamber. One of the auxiliary air compartments shown also includes an optional auxiliary burner 46 for further control of the stabilization of combustion at low coal loads. The primary air, fuel air, and auxiliary air are directed into the combustion chamber generally tangentially to an imaginary circle within the combustion chamber for the creation of a fireball therein like that shown in FIG. 1.
While the furnace of FIG. 1 includes SOFA compartments, other conventional but older furnaces do not include SOFA compartments and exhibit higher NOx emissions. Such a conventional but older furnace is shown, for example, in FIG. 3, and includes four windboxes 48 each installed in a corner of the furnace, only one of which is shown for clarity of illustration. Each windbox 48 includes, vertically spaced therein from bottom to top thereof: a first auxiliary air compartment 52 commonly referred to as an end air compartment; a first coal compartment 54; a second auxiliary air compartment 56; a second coal compartment 58; a third auxiliary air compartment 60; a third coal compartment 62; a fourth auxiliary air compartment 64; a fourth coal compartment 66; a fifth auxiliary air compartment 68; a fifth coal compartment 70; a sixth auxiliary air compartment 72; a sixth coal compartment 74; and a second end air compartment 76. Furthermore, coal nozzles 78 disposed within the coal compartments 54,58,62,66,70,74 are evenly spaced vertically apart, as is conventional. In the particular windbox 48 shown, the coal nozzles 78 are spaced 5'4" apart from one another.
Since the older furnace including windboxes 48 exhibit higher levels of NOx emissions, a retrofitting method for each windbox 48 has been developed which results in lower levels of NOx emissions. The resulting retrofitted windbox 50 is shown in FIG. 4 and includes, vertically spaced therein from bottom to top thereof: a first auxiliary air compartment 80 commonly referred to as an end air compartment; a first coal compartment 82; a second auxiliary air compartment 84; a second coal compartment 86; a third auxiliary air compartment 88; a third coal compartment 90; a fourth auxiliary air compartment 92; a fourth coal compartment 94; a fifth auxiliary air compartment 96; a fifth coal compartment 98; a sixth auxiliary air compartment 100; a sixth coal compartment 102; and, then, three consecutive closed coupled overfire air (CCOFA) compartments 104,106,108.
As will be apparent in contrasting the retrofitted windbox 50 with the older windbox 48, a coal nozzle has been removed from the top of the original windbox 48 and a coal nozzle 110 has been installed between two remaining coal nozzles 112,114 in order to accommodate the new CCOFA compartments 104,106,108. The retrofitted windbox 50 subsequently includes a spacing of 5'4" respectively between coal nozzles disposed within the first, second, third, and fourth coal compartments, but includes a spacing of only 2'8" respectively between the coal nozzles disposed within the fourth, fifth, and sixth coal compartments.
In the retrofitted windbox of FIG. 4, overall NOx emissions are reduced as a result of the diversion of a greater amount of the total air to the upper levels of the windbox 50 from the lower and middle levels of the windbox 50 that correlate with the initial and middle stages of combustion. However, it is believed that the temperature at each stage of combustion actually increases adjacent the fourth, fifth, and sixth coal compartments as a result of their necessary disposition closer to one another for accommodation of the CCOFA compartments, thereby counteracting to some extent the decreased NOx produces by the diversion of air to the CCOFA compartments.
The present invention advances the state-of-the-art of tangentially fired furnaces by providing additional improvements therein for the reduction of NOx emissions and, in particular, by providing an alternative retrofitting method to the conventional retrofitting method described above.